Steering of a vehicle includes a type with a hydraulic power assist (hydraulic power steering) and a type with an electric power assist (electric power steering).
The hydraulic power steering is the mainstream in the related art, and it is to assist a steering force with pressure oil ejected from an oil pump that operates using an engine output. The hydraulic power steering, however, has a problem that the vehicle has lower fuel economy, because part of an engine output is used as its power source.
On the contrary, the electric power steering is to assist a steering force with an electric motor that operates using electric energy from the battery. Because the electric power steering system is able to enhance fuel economy of a vehicle in comparison with the hydraulic power steering, the use of the electric power steering has been spreading in recent years.
For the steering rack of a vehicle that extends in the crosswise direction, the following are required:
(1) excellent impact resistant to the extent that no breaking occurs upon application of an impact when the vehicle runs upon the curbstone or the like;
(2) a property (breaking resistance) of not causing a breaking when bending stress is applied; and
(3) abrasion resistance of rack teeth.
For such a steering rack, S45C steel (see, for example, Japanese Unexamined Patent Publication (KOKAI) Nos. 62-178472 and 62-180018), and medium carbon steel (see, for example, Japanese Unexamined Patent Publication (KOKAI) Nos. 2000-153336 and 2001-79639) have been used in the related art. In addition, not only the abrasion resistance, but also the strength against bending stress (breaking resistance) is increased by forming a surface hardening layer through induction quenching.
For example, even when bending strength of the steering rack is increased through induction quenching, once cracking occurs in an induction quenched layer upon application of an excessively large load, the cracking develops to an inner portion and may possibly give rise to a breaking.
Further, the electronic power steering, the use of which has been spreading in recent years, tends to increase a contact surface pressure between the steering rack and the pinion gear in comparison with the hydraulic power steering, and abrasion resistance becomes insufficient when S45C steel is used. When medium carbon steel is used, the impact resistance is deteriorated even when abrasion resistance is increased by using a generous amount of C.
For example, Japanese Unexamined Patent Publication (KOKAI) No. 10-8189 discloses that in a case where induction quenching is performed with the addition of B, the resultant steel for a steering rack undergoes bending deformation without causing brittle fracture even when an excessively large load is applied. This steel for a steering rack is steel obtained by omitting quenching and tempering, and is substantially ferrite-perlite in terms of structure.
An object of the invention is to provide a steel bar for a steering rack that has excellent impact resistance while abrasion resistance is improved and is capable of preventing development of cracking, a manufacturing method of the same, and a steering rack using the same.